MPEG stands for Moving Picture Experts Group and it represents an ISO/IEC standards group developing standards for the compression of moving pictures and associated information. MPEG I and MPEG II are two such standards.
An MPEG transmission system allows several video, audio and associated services to be multiplexed and sent over a single digital transmission channel. The number of services and hence the cost of transmission bandwidth per service is determined by the bit rate. Any improvement in picture quality or reduction in bit rate is thus very important to a service provider.
Most sources of video produce random noise: camera noise, tape noise and the digital re-transmission of existing analogue services are typical examples of systems introducing noise. Although much of this noise is often biased towards the high frequency parts of the spectrum and are not particularly visible in an analogue system, MPEG encoding of such material often introduces Discrete Cosine Transform (DCT) effects or artefacts that `crawl` around the picture.
There are two main reasons for these effects being produced. Firstly, the presence of noise causes many small amplitude high frequency DCT coefficients to be generated and sent in the bit stream. These coefficients tend to be more inaccurately quantised than the low frequency coefficients and are generally due to the noise only. The increase in the number of bits transmitted causes the Quantisation Parameters factor (QP) to become higher in order to maintain the same bit rate. The net result is that the whole picture is reduced in quality. The Forward Prediction (P) and Bi-directional prediction (B) frames that follow the Intra (I) frame try to constantly correct for the noise in the prediction path and so this results in the DCT artefacts changing from frame to frame. The second reason for the loss in picture quality is that the accuracy of the motion estimation is reduced with the presence of noise in the encoder itself. This produces even worse predictions in the `B`, and `P` frames which inevitably increases the QP and reduces picture quality.
One object of the present invention is to provide a system which overcomes at least some of the limitations of the prior art and provides an improvement in performance over known systems.